1. Field of the Invention
The present invention relates generally to mass flow rate meters, and more particularly to an improved meter using Coriolis effects for measuring mass flow rates.
2. Description of the Prior Art
There has long been a need for more accurate and efficient meters for determining the mass flow rate of fluids and/or solids flowing through a pipeline or conduit. Gyroscopic or Coriolis effect type mass flow rate meters are known in the prior art. For example, U.S. Pat. No. 4,109,524 to James E. Smith for "Method and Apparatus for Mass Flow Rate Measurement" discloses a Coriolis effect meter wherein a straight conduit has first and second sections which are reciprocated by mechanical force applied through a parallelly disposed beam with its ends mechanically linked to the two sections' adjacent inside ends, which are also connected to each other through a flexible coupling. The outside ends of the two sections are fixedly supported on a base. The reciprocating forces are in addition to the Coriolis forces separately developed in opposite directions in the two sections. This induces, about the center of the beam, a moment which is measured by a torque sensor when driving acceleration forces are zero at the maximum angular velocity of the sections. This maximum velocity measurement can be complicated by spurious seismic, vibrational, or other forces which may be received through the support structure, and need to be minimized or avoided. Similar meters are disclosed in the U.S. Pat. Nos. to Wiley et al. 3,080,750; Sipin 3,218,851 and 3,329,019; and Souriau 3,396,579.
Rather than using linear sections of conduit reciprocated at adjacent ends, mass flow meters also use a curved or U-shaped tube having inlet and outlet leg ends fixed to a base, and having a bight end which is reciprocated so that corresponding side leg portions' differential displacements caused by Coriolis forces can be measured to indicate the rate of mass flow through the tube. Such a technique is suggested by Smith in U.S. Pat. Nos. '524 and 4,187,721, now RE 31,450, in which a U-shaped conduit mounted as a beam which, while driven in a no-flow condition, bends without torsional forces Thus, the torsional forces distorting or twisting the U-shaped conduit can only result from the Coriolis forces. An oscillating means, mounted on a spring arm with a natural frequency substantially equal to that of the U-shaped conduit, reciprocates the center of the loop's bight end up and down. Sensors detect the leading and trailing leg passages through the U-tube's static or oscillation midplane. According to the '450 Patent, measuring the midplane passage time difference indicates the mass flow rate while minimizing measurement of inertial acceleration and other forces resulting from oscillating the conduit: "[R]ather than compromising the accuracy of the flow meters by measuring but one of the opposing forces, the method and apparatus of the present ['450] invention is specifically structured to minimize or obviate the forces generated by the two non-measured opposing forces, i.e., velocity drag and acceleration of mass. This effort has been successful to the point where such forces are present in cumulative quantities of less than 0.2% of the torsional spring force." (col. 3, lines 5514 62). The Smith '450 patent assignee, Micro Motion, reiterates the importance of the midplane measurement and the importance of minimizing non-measured forces in its Model C Instruction Manual (page 9): "An important feature of the detection system is that deflection angle measurements are made near the center position of the tube travel, where the tube velocity and the deflection angle are the greatest. Also, at that position the angular acceleration of the tube is nearly zero, so any imbalance in the tube assembly is least likely to cause an angular deflection which might be interpreted as a flow signal." Smith U.S. Pat. No. 4,422,338 teaches a similar structure in which a pair of velocity sensors provide output signals linearly representative of the tube's actual motion, and in which electronics are provided to integrate velocity signals into position signals to permit measurement of the time difference of the side legs' passages through the oscillation midplane, or through some other selected spatial location.
In U.S. Pat. No. 4,127,028 to Bruce M. Cox for "Coriolis Mass Flow Rate Metering Means", a pair of generally U-shaped tubes are spaced apart in parallel with their inlet and outlet ends fixedly cantilever-mounted and with their respective bight ends free to move relative to each other. The tubes' respective bight ends are connected by a drive mechanism which oppositely reciprocates the tubes like tines of a tuning fork. Mass flow rate is detected by photodetectors as in the single tube embodiment of Smith '450. Cox observes that the photodetectors could be replaced by strain gauges or velocity sensors, and that the two sensor's outputs are phase shifted in proportion to the Coriolis force couple. The Cox sensors are positioned at the conduits's neutral or at-rest (midplane) position. U.S. Pat. No. 4,127,028 narrows the separation of the support ends of the legs of a U-tube to improve the ease with which the U-tubes twist torsionally in response to Coriolis reactance forces. This reference illustrates a looped tube configuration in FIG. 5, but fails to teach or suggest how such a configuration could enhance mass flow measurements, and therefore is not believed to anticipate the present invention.
U.S. Pat. No. 4,311,054 to Cox and Ho also uses sensors with linear response characteristics in a narrow region about the oscillation midpoint to measure the time difference of opposite sides passing through the loop's static (or oscillation) midplane. Other prior art Coriolis principle meters are taught or cited in the U.S. Pat. Nos. to Barnaby et al. 2,752,173; Roth 2,865,201 and 3,049,919; Sipin 3,355,944 and 3,485,098; Catherall 3,955,401; Shiota 4,381,680; and Smith RE 31,450.
If one were to employ the Smith '338 Smith '450, Cox '054 or Cox '028 teaching with a conduit that is of a non-planar configuration, the technique of measuring the conduit position relative to its rest or oscillation "midplane" position would result in mass flow measurement errors because other forces (in addition to the Coriolis forces) may tend to angularly distort the conduit and thus distort mass flow measurement. Such angular distortions can result from temperature, fluid hydrostatic pressure, or fluid flow induced centrifugal forces. In addition, the driving means or the three-dimensional nature of the conduit can introduce forces which contribute angular distortions about the conduit rest position, which can be misinterpreted as effects of Coriolis forces.
In the Smith '338 Patent if the velocity signals are not highly linear throughout the tubes entire range of motion, significant errors would result in the determination of mass flow rates.
A disadvantage of the Smith and Cox type of prior art mass flow meters is that they are highly sensitive to external vibrations (other than Coriolis forces) which may interfere with accurately measuring mass flow rates.
Another disadvantage of the Smith RE 31,450 and 4,422,338 preferred embodiments is that their methods of measuring U-tube midplane passage times produces errors during changes in the flow density.
Yet another disadvantage of prior art Coriolis type meters is that their sensitivity limits prevent them from providing accurate flow data over wide flow ranges.
Yet another disadvantage of prior art Coriolis type meters is that they incorrectly and only approximately depend upon mass flow rates being directly proportional to differential phase angles or differential time measurements.
A further disadvantage of prior art Coriolis type meters is that sensing-structure temperature changes can cause substantial errors in measuring mass flow rates.
Still another disadvantage of the prior art Smith U.S. Pat. Nos. RE 31,450 and 4,422,338 and Cox, Gonzales U.S. Pat. No. 4,127,028 is that the oscillatory drive motion creates significant bending stresses at the attachment points of the U-tubes, leading to a danger of stress corrosion.
Still another disadvantage of the above prior art meters is that they introduce at least two sharp 90 degree bends in the inlet and outlet portions of the flow meter, causing a greater loss in pressure.